The present invention relates to a circuit and method for driving a liquid-crystal display, such as an active-matrix display employing thin-film transistors.
Active-matrix liquid-crystal displays or LCDs are widely employed in, for example, portable computers, where they provide the advantages of high-speed response and reduced crosstalk. In a typical active-matrix LCD, for each of the three primary colors, each picture element or pixel has a thin-film transistor (TFT) that is switched on and off by a signal received from a gate line, and a liquid-crystal capacitor that charges or discharges through a source line when the TFT is switched on. The source and gate lines form a matrix in which the gate lines are activated one at a time, and the source lines carry signals representing the displayed intensities of the picture elements. In the alternate-current or AC driving system that is usually employed with active-matrix LCDs, adjacent liquid-crystal capacitors are charged in opposite directions centered around a common potential. A more detailed description of the active-matrix circuit configuration and AC driving scheme will be given later.
In the AC driving scheme, as successive gate lines are activated, the voltage of each source line must swing alternately above and below the common potential. The voltage swings on the source lines are thereby doubled, as compared with direct-current driving. A resulting problem is that the time needed to charge the parasitic capacitances of the source lines is increased, current consumption is similarly increased, and large source-line driving circuits are needed. The large charging and discharging currents furthermore generate electrical noise.
Although the gate lines are not driven in an AC manner, they also have parasitic capacitances that must be charged and discharged. The charging and discharging of the gate lines similarly takes time, consumes current, generates noise, and requires large driving circuits.
It is accordingly an object of the present invention to reduce the current consumed in charging the parasitic capacitances of signal lines in a liquid-crystal display.
Another object of the invention is to reduce the time needed for charging the parasitic capacitances of signal lines in a liquid-crystal display.
Yet another object is to reduce electrical noise in a liquid-crystal display.
Still another object is to reduce the size and cost of the driving circuits of a liquid-crystal display.
The invented driving circuit drives a liquid-crystal display having first signal lines running in one direction, second signal lines running in another direction, switching elements controlled by the first signal lines, disposed at the intersections of the first and second signal lines, and liquid-crystal capacitors disposed at these same intersections and coupled through the switching elements to the second signal lines.
The driving circuit comprises a plurality of first drivers for sequentially driving the first signal lines to active and inactive levels, thereby switching the switching elements on and off, and a plurality of second drivers that drive the second signal lines with signals representing picture-element intensities.
According to a first aspect of the invention, a switching circuit is coupled to the second signal lines. At transition times when any of the first signal lines change between the active and inactive levels, the switching circuit disconnects the second signal lines from the second drivers, and places the second signal lines in a short-circuited state. The second signal lines may be short-circuited to a fixed potential, or short-circuited to each other.
According to a second aspect of the invention, a switching circuit is coupled to the first signal lines. When a pair of first signal lines changes between the active and inactive levels, the switching circuit disconnects that pair of first signal lines from the corresponding first drivers, and places that pair of first signal lines in a short-circuited state. The short-circuited pair of first signal lines may be short-circuited to a fixed potential, or short-circuited to each other.
The switching circuits in both the first and second aspects of the invention may incorporate resistors to limit the peak current flow on the short-circuited signal lines.
According to a third aspect of the invention, the second drivers drive each of the second signal lines alternately above and below a certain center potential. Each second signal line stays at potentials equal to or greater than the center potential while a plurality of first signal lines are being driven to the active level, then stays at potentials equal to or less than the center potential while another plurality of first signal lines are being driven to the active level.
The first and second aspects of the invention recycle charge from one signal line to another through the short circuits, thereby reducing current consumption, reducing electrical noise, and enabling the signal lines to be driven more rapidly, or to be driven by drivers with less driving capability, hence with smaller size and lower cost.
The third aspect of the invention provides similar effects by reducing the frequency with which the second signal lines are driven from one side of the center potential to the opposite side.